🌱

Basic

CRUD · Iteration · Comprehension · Set Operations

Week 7 — Dictionaries & Sets

Use dictionaries for key-value storage and sets for unique collections. Learn dict comprehensions, set algebra, and when to choose each structure.

dictsethashcomprehension

Duration

⏱ 2.5 hours

Level

📊 Beginner

Prerequisite

🎯 Week 6

OUTCOME

Build a word frequency counter using dictionaries

What you'll learn

1Create, read, update, and delete dictionary entries
2Iterate over keys, values, and items
3Build dict comprehensions
4Perform union, intersection, and difference on sets
5Know when to use dict vs list vs set

1. Dictionaries

Dictionaries store key-value pairs. Keys must be unique and hashable (usually strings or numbers).

python

person = {"name": "Alice", "age": 25, "city": "Seoul"}
print(person["name"])          # Alice
person["email"] = "a@ex.com"  # add
person["age"] = 26             # update
del person["city"]             # delete
print("age" in person)         # True
print(person.get("phone", "N/A"))  # N/A (safe access)

2. Iterating Dictionaries

python

scores = {"Alice": 95, "Bob": 82, "Charlie": 78}
for name in scores:            # keys
    print(name)
for score in scores.values():  # values
    print(score)
for name, score in scores.items():  # both
    print(f"{name}: {score}")

3. Sets

Sets hold unique, unordered items. Great for membership tests and set algebra.

python

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
print(a | b)  # union:        {1,2,3,4,5,6}
print(a & b)  # intersection: {3,4}
print(a - b)  # difference:   {1,2}
print(a ^ b)  # symmetric diff: {1,2,5,6}

4. Dict Comprehension

python

# Square each number
squares = {n: n**2 for n in range(1, 6)}
print(squares)  # {1:1, 2:4, 3:9, 4:16, 5:25}

# Filter: only even keys
even = {k: v for k, v in squares.items() if k % 2 == 0}
print(even)  # {2:4, 4:16}

5. Common Mistakes

dict[key] on a missing key → KeyError. Use dict.get(key, default) for safe access.
Sets are unordered — don't rely on insertion order. Use a list if order matters.
{} creates an empty dict, not a set. Use set() for an empty set.

💻 Examples

Run these examples and check the output yourself.

01_phonebook.py— Phone book with CRUD

CODE

phonebook = {}
while True:
    cmd = input("add/lookup/delete/quit: ").strip()
    if cmd == "add":
        name = input("  Name: ")
        phone = input("  Phone: ")
        phonebook[name] = phone
        print(f"  Saved {name}.")
    elif cmd == "lookup":
        name = input("  Name: ")
        print(" ", phonebook.get(name, "Not found"))
    elif cmd == "delete":
        name = input("  Name: ")
        phonebook.pop(name, None)
    elif cmd == "quit":
        break

02_word_count.py— Word frequency counter

CODE

text = input("Enter a sentence: ").lower()
words = text.split()
freq = {}
for w in words:
    freq[w] = freq.get(w, 0) + 1
for word, count in sorted(freq.items(), key=lambda x: -x[1]):
    print(f"  {word}: {count}")

03_set_ops.py— Set algebra on two word lists

CODE

a = set("the cat sat on the mat".split())
b = set("the dog sat on the log".split())
print("Common words:  ", a & b)
print("Unique to A:   ", a - b)
print("All words:     ", a | b)

▶ Output

Common words:   {'the', 'on', 'sat'}
Unique to A:    {'cat', 'mat'}
All words:      {'the', 'on', 'sat', 'cat', 'mat', 'dog', 'log'}

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Student Grade Book

Goal: Store student names and scores in a dict; print average, highest, and lowest.

Requirements

Read N students from input
Store in dict {name: score}
Print average (2 decimal), top scorer, lowest scorer

▶Toggle solution

SOLUTION

n = int(input("Students: "))
grades = {}
for _ in range(n):
    name = input("  Name: ")
    score = int(input("  Score: "))
    grades[name] = score
avg = sum(grades.values()) / len(grades)
top = max(grades, key=grades.get)
low = min(grades, key=grades.get)
print(f"Average: {avg:.2f}")
print(f"Top: {top} ({grades[top]})")
print(f"Lowest: {low} ({grades[low]})")

Exercise 2

Duplicate Detector

Goal: Find duplicate numbers in a list using a set.

Requirements

Read space-separated integers
Use a set to track seen numbers
Print duplicates in sorted order

Sample I/O

Numbers: 3 5 2 3 7 5 9
Duplicates: [3, 5]

▶Toggle solution

SOLUTION

nums = list(map(int, input('Numbers: ').split()))
seen, dups = set(), set()
for n in nums:
    if n in seen:
        dups.add(n)
    seen.add(n)
print('Duplicates:', sorted(dups))

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗